GLOBIN GENE SWITCHING IN TRANSGENIC MICE CARRYING HS2-GLOBIN GENE CONSTRUCTS

We have examined the pattern of human globin gene switching in transge nic mice containing three different gamma and beta gene constructs (HS 2(G) gamma(A) gamma delta beta, HS2(A) gamma beta neo, and HS2(A) gamm a en beta) and compared the results with previously described transgen ics (HS2(A) gamma beta, HS2(G) gamma(A) gamma(-117)delta beta, and LCR epsilon(G) gamma(A) gamma delta beta). Developmental regulation was o bserved in all cases with identical patterns in lines bearing the same construct. Three different patterns of switching were observed: LCR e psilon(G) gamma(A) gamma delta beta and HS2(A) gamma delta neo mice sw itched rapidly, HS2(G) gamma(A) gamma delta beta and HS2(G) gamma(A) g amma(-117)delta beta at an intermediate rate, and HS2(A) gamma beta an d HS2(A) gamma en beta mice showed delayed switching, with a plateau i n late fetal-early neonatal life and readily detectable levels of gamm a mRNA in adults. No difference was observed in the time of switching of the HS2(G) gamma(A) gamma delta beta mice compared with those with the (A) gamma(-117) hereditary persistence of fetal hemoglobin mutatio n, but adult levels of gamma mRNA were significantly higher (approxima te to 5%) in lines carrying the mutation than in those without (approx imate to 1%). Reversion to the rapid switch of the LCR epsilon(G) gamm a(A) gamma delta beta mice was observed in three lines with the HS2(A) gamma beta neo construct in which expression of the tk-neo gene was a pproximately equal to that of the globin genes, The inclusion of the ( A) gamma enhancer in HS2(A) gamma beta mice did not alter the pattern of switching, or reduce the relatively high levels of gamma mRNA in th ese lines. However, unlike other HS2 mice, the combination of HS2 and the (A) gamma enhancer resulted in copy number-dependent expression in HS2(A) gamma en beta lines, with intrauterine death at approximate to 12.5 days gestation at high copy numbers. These results demonstrate t hat numerous elements throughout the beta globin gene cluster interact to produce the correct pattern of developmental regulation of these g enes. Furthermore, extinction of gamma gene expression in adult life i s not completely autonomous and is incomplete when HS2 is the only LCR element present. (C) 1997 by The American Society of Hematology.